This error is caused by an Apache configuration limitation on the “LimitRequestBody” setting. To resolve this issue you should ask your host to increase this value to anything above 1 MB.
Alternatively, you could try increasing this value yourself by adding the following to your site’s root .htaccess file.
You can also try lowering the “Maximum Request Size” setting using the slider control on the settings tab of the Migrate DB Pro plugin page. The minimum size is 512 kB.